I am upgrading a Cambridge R50 crossover.
I implemented the 18 dB B139 branch (7mH, 3,5mH, 80uF), wich improves midrange clarity by reducing higher breakup modes from the B139.
I implemented the 18 dB well known aB part for the T27, removing the annoying fc ringing around 1200 Hz, and also giving a much less harsh sound, very pleasing indeed.
(I have discovered that very minor balance adjustments (0,5 dB) make a huge difference, not so much in treble loudness, but in treble integration, and thus treble quality)

My question: I want to improve the B110A midrange in the same 18 dB-way.
I have a crossover layout (From HH Klinger? early eighties?), as follows:
1mH and 30uF in series, then 10uF and 4mH in parallel to earth, then 0,4mH in series (thus forming a 18db lowpass and a 12 dB highpass), then a 7uF+10R zobel to earth, and then (?!) a 5R parallel to 16uF to the B110A.
(Sorry, I don't know how to upload a *.jpg with the layout)

This last series part (5 Ohm parallel to 16uF), and these in series with the driver puzzle me. Sounds awful, omitting them helps a lot. Is it a print error? Has the 16uF to be a 60?160?uF, (would make sense for a 18dB highpass)? With the 5 Ohm and 16uF there is not much mid left (not loud enough, loss of lots of lower mids)
Is there anyone who has experience with this layout?

There are two ways of finding out what the crossover filter is doing; measurement and simulation. Apply (1V?) tones to the input of the crossover and measure the voltage across the drive unit. Change the crossover and re-measure. Repeat the process as required. Simulation is quicker and involves no soldering but you will need an accurate measurement of the drive unit impedance in the first place, to use as a load for the simulator.

Speakerguru, thank you for the reply. Yes, I have some equipment to my disposal te explore crossover behaviour in the non-digital way. However, I will get then voltage outputs from the filter, probably non-linear, from which I cannot deduce if that leads to a linear acoustic target response.
Yes, I have some clue what the mid-response curves have to look like, from KEF Topics and the constructor series leaflets, but that will only be 'about right'. And I want to be shure.

Simulation: yes I'd like to try, but only useful if I could find the right component R,L,C(etc)-values of the B110A itself (B110 subtitution diagram).
Which simulation program would you advise?

The design at hand is probably a KEF lay-out, (Falcon offers an improvement for the DN12 3-wayXO, derived from this crossover).
It has strong similarities with the DN19/20 Cantata 18 dB crossover (for B110B and T52), so I think KEF worked this DN19/20 over for the B110A and the T27, and probably came to life in the same era.

My question is if someone knows the correct component values in the B110A branch in this XO, probably someone who owns a KEF speaker with the genuine filter (IF it exists?).

Hello Kimmo, Thanks. Yes, this might be it. So I googled and looked on this forum for falcon no 53 and /or Auskef (?), but could only find threads speaking about it (reinforcing the idea that this is the actual XO), but nowhere the actual layout with component values. Any hints? Has it ever been published?

Hello Proffski, thanks so far. Difficult to find indeed. I have a *jpg from years back that I can mail from the XO that I wish to check, maybe the Auskef (how mail/attach jpg's? new on the block). When young & energetic I tried most of the designs you mentioned:

Atkinson: great kit. Don't follow the red lines, it spoils the 'baffle-step-avant la lettre' designed for the B110A. Sounded very smooth mid+high, and the deepest bass fom the B139 I heard in my room (27Hz) (Haven't tried a big IMF though, only compared to PRO9TL and R50 and Bailey's, they stopped at 31Hz in my room and other, rather small, rooms)

Webb / R50 great, considering the low number of XO components, but this is what I want to update. I like the relative clarity and simplicity of the design, in an acceptable package.

105: I did a kind-of-rebuild with the CS9 components in a big TL but with the the PL301 rounded-off front, for better dispersion, and tried both 105 (I&II) versions. Very stable performance and stereo image regardless of input power. But under normal listening levels I prefer the crispness of the T27 (the 105-TL was way too big anyhow).
So that's what I'm after: a full 18 dB B139 B110 T27 XO.

When I built my TLs years ago in the end I settled with 24dB/octave crossovers designed for B139, B110, and Decca London ribbon tweeter.
Still being used by a friend after all these years. The monsters are all fully Tri-Amped. More description if you click on the pictures.
I learnt a lot building these and also found out how little I knew about loudspeaker design, but I was young and foolish!

Wow! Great speakers. What appeals is the strict black form without any frills and frippery, aesthetic golden shine on the B139 (after the bottle possibly perceived as sunset on a strange planet), 'just' a sensible three way, Decca ribbons. Must have been a great satisfaction after all the work! I fear Its not possible to smuggle these in the home (although some slow process stretching of speakersize is creeping in)

Never again! I found out the hard way how little I knew about loudspeaker design! And yes, in the end it was worth it. _________________I contend that for a nation to try to tax itself into prosperity is like a
man standing in a bucket and trying to lift himself up by the handle.
-Winston Churchill